Apparatus, method and system for control of ac/ac conversion

ABSTRACT

A method, system and apparatus for controlling a pulse width modulator (PWM) converter for direct AC/AC conversion and/or AC voltage regulation. According to some embodiments of the invention, an output voltage may be provided, independent of the input voltage quality, thereby avoiding or minimizing power company irregularities, brownouts and the like. Embodiments of the present invention may be useful, for example, for use in connection with motors and motored devices or other applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/097,200, filed Oct. 14, 2008, which is a National PhaseApplication of PCT International Application No. PCT/IL2006/001420,International Filing Date Dec. 11, 2006, claiming priority of U.S.Provisional Patent Application No. 60/749,045, filed Dec. 12, 2005, allof which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The field of this invention is generally power conversion, and morespecifically, power conversion and/or voltage regulation for AC/ACapplications.

BACKGROUND OF THE INVENTION

In some alternating current (AC) electrical systems it may be beneficialto provide for stabilization of the output voltage. Known devices andmethods for stabilizing AC output voltage may typically requireconversion to direct current (DC) and then back to AC, which may resultin inefficiency and high production costs. Other devices may usevariacs, electromechanical devices, and/or components withferro-resonance characteristics. Yet other systems may use or include anuninterruptible power supply (UPS), however, the UPS may be too largeand/or expensive for many applications that may benefit from stabilizedor regulated AC output voltage. Accordingly, there is a need for anefficient and inexpensive method and device for AC voltagestabilization.

FIG. 1 illustrates a step-down synchronous buck converter arrangement100, also referred to as an electronic transformer or line conditioner.In operation, AC source 101 may provide an input voltage signal. Thevoltage at reference node 102, which may be a signal voltage-dividedfrom the input voltage signal, may be fed to pulse width modulator (PWM)module 108. The operation may use a synchronous PWM controller 108 toprovide the control signals for switches 103 and 104, via signals atoutputs Qa and Qb, where Qa and Qb are substantially complimentary.Switches 103 and 104 may be controlled by the PWM or synchronousconverter 108 to chop the input voltage. Inductor 105 and capacitor 106comprise a basic output filter that may filter the voltage and providethe load 107 with stable AC voltage. Accordingly, based on the pulsewidth modulation, which drives the switching scheme, load 107 may beprovided with an output voltage amplitude that is less than the inputvoltage.

FIGS. 2( a), 2(b), 2(c) and 2(d) illustrate examples of implementationsof bidirectional switches that may be used in connection with an AC/ACconverter, including using field effect transistors (FET) and bipolarjunction transistors (BJT), as well as diodes.

In the example provided in FIG. 1, the output voltage amplitude at load107 may be:

Vout=Vin×DC,  (1)

where DC represents the duty cycle of signal Qa, e.g., the time Qa isconducting as a fraction of the total period of the signal.

In the configuration of FIG. 1, due to the output filter constructedusing inductor 105 and capacitor 106, the output voltage will be delayedrelative to the input voltage, and therefore, the output voltage may beout of phase with the input voltage signal, thereby producing harmonicdistortion and/or phase distortion. Since the control loop of thecircuit configuration may be referenced to the input voltage signal, thecircuit will try to obtain an output voltage signal in phase with theinput voltage signal, which, as described in further detail below, maycause distortion at or near the zero crossing of the input voltage, asseen at FIG. 3, below.

FIG. 3 is a graph 300 plotting input voltage 320 and output voltage 330along time axis 310 for a circuit such as the one depicted in FIG. 2.Output voltage amplitude may be less than that of the input voltageamplitude by a factor equal to the duty cycle, e.g., 50% for a dutycycle of 50%. The output voltage may be delayed or out of phase withrespect to the input voltage by t=t, where t may be determined by thecharacteristics of the output filter, for example, the inductance andcapacitance values of the output filter.

When using a PWM regulator for line conditioning applications, theoutput voltage may be phase-locked to the input voltage, for example, inorder to achieve smooth transitions in the case of bypass conditioningand small phase margins between the three phase circuits. In some caseswhere output voltage must be in phase to input voltage, a closed-loopcontrol is appropriate. Closing the control loop for zero delay outputvoltage with respect to input voltage, however, may result in the dutycycle demand as shown in FIG. 4.

FIG. 4 depicts a graph 400 of the duty cycle 420 varying along time axis410 that would be required in order to provide for an output voltagehaving no phase delay with respect to input voltage, where Equation (1)is rewritten as DC=Vout/Vin. In the case of closed-loop control, aportion of the output voltage is sensed and compared to the inputvoltage to produce an error voltage for the control loop. As seen withrespect to the graph of FIG. 4, the required duty cycle may approachpositive infinity 430 just before the zero crossing of the input voltageand re-appear at negative infinity 440 just after the zero crossing ofthe input voltage signal. Such demands may produce clipping at the highand low boundaries of the feasible duty cycle, e.g., 100% and 0%. Inaddition, in real-world applications, it is difficult if not impracticalfor the control loop to handle an instantaneous change from positivelyinfinite required duty cycle to a negatively infinite required dutycycle, or, for example, 100% duty cycle to 0% duty cycle. Accordingly,the output voltage may contain errors and total harmonic distortion(THD) may result.

SUMMARY OF THE INVENTION

In a converter for converting an input alternating current (AC) signalto an output AC signal using synchronous pulse width modulation, oneembodiment of the invention may include an apparatus for providingswitching signals to at least first and second converter switches,comprising a selectable crossover module adapted to receive first andsecond input signals produced by a pulse width modulator, and to providesignals at first and second outputs to the first and second converterswitches, the crossover module having first and second modes, wherein inthe first mode, the first output of the crossover module is connected toprovide the first input signal, and the second output is connected toprovide the second input signal, and in the second mode, the firstoutput of the crossover module is connected to provide the second inputsignal, and the first output is connected to provide the second inputsignal. In some embodiments of the invention, the crossover module maybe further to receive a control signal for selecting between the firststate and the second state. In some embodiments, there may be means forproviding the control signal to change modes of the crossover modulewith reference to a change in polarity of a control reference signal. Insome embodiments the control reference signal may be the voltage of theinput AC signal, the voltage of the output signal, or a combinationthereof. Embodiments of the invention may further include a pulse widthmodulation controller to provide the first and second input signals, thefirst and second input signals having substantially complementary dutycycles determined by a level of an input reference signal. In someembodiments of the invention, the input reference signal may be derivedby dividing an error signal by the input AC signal, wherein said errorsignal based on a difference between the input AC signal and the outputAC signal.

In a converter for converting an input alternating current (AC) signalto an output AC signal using synchronous pulse width modulation,embodiments of the invention may further include an apparatus forproviding switching signals to at least first, second, third and fourthconverter switches, comprising a second selectable crossover moduleadapted to receive first and second input signals produced by a secondpulse width modulator, and to provide signals at first and secondoutputs to the third and fourth converter switches, the second crossovermodule having first and second modes, wherein in the first mode, thefirst output of the second crossover module is connected to provide thefirst input signal, and the second output is connected to provide thesecond input signal, and in the second mode, the first output of thesecond crossover module is connected to provide the second input signal,and the first output is connected to provide the second input signal.Embodiments of the invention may further include a second pulse widthmodulation controller to provide to said second selectable crossovermodule the first and second input signals, the first and second inputsignals having first and second respective duty cycles determined by alevel of an input reference signal, and being substantiallycomplementary to each other. In one embodiment of the invention, one ofthe pulse width modulation controllers may be a buck controller, and theother pulse width modulation controller may be a boost controller.

In a converter for converting an input alternating current (AC) signalto an output AC signal using synchronous pulse width modulation, anembodiment of the present invention may include an apparatus forproviding switching signals to at least first and second converterswitches, comprising a selectable crossover module adapted to receive aninput reference signal and provide an output signal to a pulse widthmodulation controller, the crossover module having first and secondmodes, wherein in the first mode, the output signal of the crossovermodule is connected to provide an output signal reference levelproportional to said input reference level, and in the second mode, thefirst signal of the crossover module is connected to provide an outputsignal reference level inversely proportional to said input referencelevel. In some embodiments of the invention, the crossover module mayfurther be to receive a control signal for selecting between the firststate and the second state. Embodiments of the invention may furtherinclude means for providing said control signal to change modes of thecrossover module with reference to a change in polarity of a controlreference signal. In some embodiments of the invention, the controlreference signal may be based on at least one signal selected from thevoltage of the input AC signal and the voltage of the AC output signal,or a combination thereof. In some embodiments of the invention, theselectable crossover module may be a selectably inverting/non-invertingamplifier. In some embodiments of the invention, the input referencesignal may be derived by dividing an error signal by the input ACsignal, wherein the error signal based on a difference between the inputAC signal and the output AC signal.

In a converter for converting an input alternating current (AC) signalto an output AC signal using synchronous pulse width modulation, theinvention may include an apparatus for providing switching signals to atleast first and second converter switches, comprising a selectablecrossover module adapted to receive an input reference signal and a rampsignal, and to provide signals at first and second outputs to a pulsewidth modulator, the crossover module having first and second modes,wherein in the first mode, the first output of the crossover module isconnected to provide the input reference signal, and the second outputis connected to provide the ramp signal, and in the second mode, thefirst output of the crossover module is connected to provide the rampsignal, and the first output is connected to provide the input referencesignal. In some embodiments of the invention, the crossover module maybe further to receive a control signal for selecting between the firststate and the second state. Embodiments of the invention may furtherinclude means for providing said control signal to change modes of thecrossover module with reference to a change in polarity of a controlreference signal. In some embodiments of the invention, the controlreference signal may be based on at least one signal selected from thevoltage of the input AC signal and the voltage of the AC output signal.Some embodiments of the invention may further include a pulse widthmodulation controller to provide first and second substantiallycomplementary switching signals having duty cycle based on the first andsecond outputs of the crossover module. Some embodiments of theinvention may further include a second selectable crossover moduleadapted to receive an input reference signal and a second ramp signal,and to provide signals at first and second outputs to a second pulsewidth modulator, the second crossover module having first and secondmodes, wherein in the first mode, the first output of the crossovermodule is connected to provide the input reference signal, and thesecond output is connected to provide the second ramp signal, and in thesecond mode, the first output of the crossover module is connected toprovide the second ramp signal, and the first output is connected toprovide the input reference signal. Some embodiments of the inventionmay further include a to second pulse width modulation controller toprovide first and second substantially complementary switching signalshaving duty cycle based on the first and second outputs of the secondcrossover module. In some embodiments of the invention, one of the pulsewidth modulation controllers may be a buck controller, and the other ofthe pulse width modulation controllers may be a boost controller. Insome embodiments of the invention, the input reference signal may bederived by dividing an error signal by the input AC signal, wherein theerror signal based on a difference between the input AC signal and theoutput AC signal.

A method of converting an input alternating current (AC) signal to anoutput AC signal using synchronous pulse width modulation in accordancewith embodiments of the present invention may include receiving an inputalternating current signal, producing a reference signal based on thelevel of the input signal, producing a pulse-width modulated signalbased on the level of said reference signal, and inverting the pulsewidth modulated signal timed with reference to a zero crossing of saidinput signal.

Embodiments of the present invention may include systems usingapparatuses or methods described herein. Systems in accordance with thepresent invention may include the apparatus for regulating orcontrolling voltage an electrical appliance, wherein the apparatus isconfigured to receive an input voltage and provide a stabilized outputvoltage to the appliance. In some systems, the apparatus of the presentinvention may be used for correcting power factor of the output voltage.In some embodiments of the present invention, the apparatus of thepresent invention may be used for regulating a voltage for a light bulbor other lighting element. In some embodiments of the invention, asystem may use the apparatus of the present invention as an AC/ACtransformer having variable output to input voltage ratio, wherein insome embodiments, the variation in ratio may be manual, while in otherembodiments, the variation may be controlled by a closed feedback loopor by an open feedback loop control.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

to FIG. 1 is a schematic drawing of a buck converter;

FIGS. 2( a), 2(b), 2(c) and 2(d) illustrate examples of implementationsof bidirectional switches that may be used in connection with an ACconverter;

FIG. 3 is a graph showing output voltage and input voltage in theconverter of FIG. 1;

FIG. 4 is a graph showing the theoretical duty cycle of an AC/AC PWMloop control;

FIG. 5 is a schematic illustration of an AC/AC converter in accordancewith embodiments of the present invention;

FIG. 6 is a graph showing duty cycle of an AC/AC PWM loop control inaccordance with embodiments of the present invention;

FIGS. 7A, 7B, 7C and 7D are schematic drawings of examples ofimplementations of crossover modules that may be used in accordance withembodiments of the present invention;

FIGS. 8A and 8B are examples of an AC/AC converter control apparatusesin accordance with embodiments of the present invention utilizing abuck-boost topology;

FIG. 9 is a schematic illustration of a crossover module arranged at theinput to a pulse width modulation controller in accordance with anembodiment of the present invention;

FIG. 10 is a schematic illustration of a crossover module arranged atthe input to a pulse width modulation controller in accordance with anembodiment of the present invention;

FIG. 11 is a schematic drawing of a PWM control loop in accordance withembodiments of the present invention;

FIGS. 12A and 12B are schematic drawings of PWM control loops inaccordance with embodiments of the present invention;

FIG. 13 is a schematic diagram of an AC/AC converter in accordance withembodiments of the present invention; and

FIGS. 14A, 14B and 14C are schematic block diagrams of applicationsutilizing embodiments of the present invention that may be used forpower factor correction.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, and components have notbeen described in detail so as not to obscure the present invention.

Embodiments of the invention may employ a method and device forcontrolling a pulse width modulator (PWM) converter for direct AC/ACconversion and/or AC voltage regulation. According to some embodimentsof the invention, an output voltage may be provided, independent of theinput voltage quality, thereby avoiding or minimizing power companyirregularities, brownouts and the like. Embodiments of the presentinvention may be useful, for example, for use in connection with motorsand motored devices or other applications. Embodiments of the presentinvention may be used to avoid, overcome or otherwise reduce asingularity problem during the zero voltage crossing of the inputvoltage signal. Embodiments of the present invention for AC/ACconversion may be used, for example, in conjunction with any of thewell-known topologies, including but not limited to buck, boost,buck-boost, Cuk, sepic, and matrix converters, or hybrids thereof. Thecontrol loop of embodiments of the invention may be used for examplewith isolated or non-isolated AC/AC converters, to eliminate or reducethe “near zero” error of output voltage for generating an output withreduced total harmonic distortion (THD). Embodiments of the inventionmay be used, for example, to improve efficiency, or to “clean” powerproblems caused by electrical motors and/or motor drives.

FIG. 5 illustrates a step-down synchronous buck converter arrangement500 in accordance with an embodiment of the present invention. Inoperation, AC source 501 may provide an input voltage signal. An inputreference voltage, which may be a signal scaled or voltage-divided fromthe input voltage signal produced by source 501, may be fed to pulsewidth modulator (PWM) module or controller 508 at node 502. In theembodiment depicted, PWM controller 508 may be a synchronous PWMcontroller to provide substantially complementary signals having dutycycles related to the level of the voltage at reference node 502. Thus,when a first output of PWM is on, the second output is off, and viceversa. Output signals of PWM controller 508 may substantiallycomplementary, for example, there may be a blanking interval to ensurethat the switches are never on simultaneously, thereby short circuitingthe output stage. Although the blanking circuitry may not be shown, itwill be recognized that such circuitry may be included for suchpurposes. Thus, for example, a first output of PWM 508 may have a smallduty cycle, e.g., 30%, for a low input reference voltage, and the secondoutput of PWM 508 may have a substantially complementarily large dutycycle, e.g., 70%.

In the embodiment shown, the outputs of PWM controller 508 may beconnected to respective inputs of a crossover switch module 510.Crossover switch module may have at least two states, a normal and acrossed state. In a normal state, the first output may be substantiallythe same as the first input, and the second output may be substantiallythe same as the second output. In a crossed state, the first output maybe substantially the same as the second input, and the second output maybe substantially the same as the first input. First and second outputsof crossover switching module 510 may be connected to switches 503 and504, respectively. A control signal to crossover switch 490 may beprovided to change the state of the crossover switch between the normaland crossed states. An output filter may be provided, for example,inductor 505 and capacitor 506, and the load 507 may be delivered with astable AC output voltage.

In embodiments of the invention, the control signal to the crossoverswitch may be timed to induce the change of state at or around the timewhen input voltage signal changes polarity. In some embodiments, thecontrol signal may be provided by a polarity detector to detect a changein polarity of the input signal. In other embodiments, the controlsignal may depend on a change in polarity of the output voltage signal.In some embodiments, the control signal may trigger the crossover at atime related to both the change of polarity of the input signal andchange of polarity of the output signal, for example at a timetherebetween. In some embodiments, the control signal may depend on anoutput of a phase detector detecting a difference in phase between theinput voltage and the output voltage.

In operation, the arrangement of FIG. 5, or other arrangements describedin connection with embodiments of the invention, may avoid or reduce thezero crossing problem, for example, by switching the inputs at, near, orapproximately the time of the zero crossing of the input signal.

FIG. 6 depicts a graph 600 of the duty cycle 620 varying along time axis610 that may be used to provide for an output voltage 630 having no orlittle phase delay with respect to input voltage 640 in an arrangementsuch as one depicted in FIG. 5 in accordance with embodiments of thepresent invention. Based on the changing state of the crossover switch,the singularity at the zero input voltage crossing may be solved, e.g.,by avoiding the requirement of the duty cycle to migrate from a largepositive duty cycle to a large negative duty cycle at the zero inputvoltage crossing. According to embodiments of the invention, the dutycycle may increase to a large positive value before the zero inputsignal crossing, remain at a large positive value, and decrease from thelarge positive value after the zero crossing of the input signal. Asimilar analysis holds true with required changes at large negativevalues of they duty cycle.

Embodiments of the invention may use various suitable implementations ofone or more crossover switches. In some embodiments, crossover switchesmay have low impedance, for example, by construction using field effecttransistors (FETs). For example, some embodiments of the invention mayuse a discrete or integrated circuit such as ADG452 manufactured byAnalog Devices. Crossover switch module may be implemented in a varietyof ways, for example, using digital logic gates, digital or analogmultiplexers, analog amplifiers, or other components.

FIG. 7A depicts a schematic diagram of a crossover module that may beused in accordance with embodiments of the invention. Inputs Qa and Qbmay be directed to first and second outputs in normal or crossed orderdepending on the state of the switch, which may be controlled by asingle control signal. FIG. 7B depicts an alternate arrangement of thecrossover module using inverters that may be used when selectivelycrossing substantially complementary signals. When the switch is in afirst state, the inputs Qa and Qb are output in non-inverted state,whereas when the switch is in a second position, the inputs Qa and Qbmay be inverted, thereby effectively providing the crossed outputs. Itwill be understood that implementation of the crossover module maydepend on the placement of the module in the circuit, as well asconsiderations of design, cost, performance, etc. FIG. 7C depicts aschematic example of a logic gate implementation of a crossover switchthat may be used in connection with the present invention. FIG. 7Ddepicts an example of a multiplexer embodiment of a crossover switchthat may be used in connection with the present invention.

It will be noted that embodiments of the invention may use more than onecrossover switching arrangements, depending on the converterconfiguration. In an embodiment of the invention depicted in FIG. 8A, acrossover switching arrangement 800 according to embodiments of thepresent invention may include two crossover modules in conjunction witha buck-boost arrangement, for example, having an H-topology. The PWMcontroller of the present invention may be integrated or used inconjunction with a buck-boost controller, for example, LinearTechnology's LTC 3780 High Efficiency, Synchronous, 4-Switch Buck-BoostController, the specification of which is incorporated herein byreference. It will be noted that the invention is not limited withregard to converter topology; buck-boost, buck, boost, Cuk, sepic,flyback, or any of an assortment of other converters may be used.Buck-boost is used here only by way of example.

In the embodiment depicted in FIG. 8A, a buck ramp generator (not shown)may provide a buck ramp signal to an input of buck comparator 801.Another input to comparator 801 may be a signal indicating the dutycycle required, for example, an input reference signal based on theinput voltage signal. The result of buck comparator 801 may provide asignal having a desired duty cycle. A blanking circuit 802 may receivethe output of buck comparator 801 and produce two signals based thereon,having substantially complementary duty cycles. These outputs may be fedto crossover switch 803, having a control signal, for example, to changestates of the crossover switch 803 based on a zero crossing of the inputreference signal, or another suitable signal or combination of signalsconsistent with the teachings of the invention. The outputs of buckcrossover switch 803 may be used to drive the buck converter switches Aand B.

With respect to the boost portion of the embodiment depicted in FIG. 8A,a boost ramp generator (not shown) may provide a buck ramp signal to aninput of boost comparator 811. Another input to boost comparator 811 maybe a signal indicating the duty cycle required, for example, an inputreference signal based on the input voltage signal. The result of boostcomparator 811 may provide a signal having a desired duty cycle. Ablanking circuit 812 may receive the output of boost comparator 811 andproduce two signals based thereon, having substantially complementaryduty cycles. These outputs may be fed to crossover switch 813, having acontrol signal, for example, to change states of the crossover switch813 based on a zero crossing of the input reference signal, or anothersuitable signal or combination of signals consistent with the teachingsof the invention. The outputs of boost crossover switch 813 may be usedto drive the buck converter switches C and D. It will be recognized thatthe crossover switch may be placed at other locations between thecomparators and the switches, for example, before the blanking circuit,after the switch control signals, or combined with any other componentin the circuit.

FIG. 8B depicts an embodiment of the invention in which the crossovermodule 820A, 820B is implemented using logic gates and placed betweenthe pulse width modulation controller and the switches. Input nodes 821,822, 823 and 824 may receive signals Qa, Qb, Qc and Qd, respectively,from a four-switch buck-boost controller. Control signal input nodes 831and 832 may receive a control reference signal, as described above. Theinverters and AND logic gates of crossover module 820A, 820B may providelogic signals to drivers 861, 862 for signal Qa, to drivers 863, 864 forsignal Qb, to drivers 865, 866 for signal Qc, and to drivers 867, 868for signal Qd. Drivers may provide driving signals to put bidirectionalswitches 841, 842, 843 and 844 into conducting/non-conducting modes asrequired by the PWM controller signals and the crossover module, so asto connect the input signal received at node 851 through inductor 850 tothe output node 852 across output capacitor 853 to obtain the outputsignal. In the embodiment of the invention depicted, the driver pairsmay have each an output node in common, for example a low node of onedriver connected to a high node of a paired driver so as to act as levelshifters. It will be recognized that any of the other bidirectionalswitches, suitably modified, may be used in accordance with embodimentsof the invention.

In some of the above embodiments, the crossover switching module wasdescribed as being implemented as a digital component, receiving andoutputting digital signals. However, embodiments of the presentinvention may place the crossover switching module at any suitablelocation in the circuit, for example, in the analog portions thereof.

FIG. 9 depicts an embodiment of the invention 900, in which thecrossover switches are placed before the boost and buck comparators.Thus, for example, buck crossover switch 901 receives the duty cyclesignal, for example, an input reference voltage signal and buck rampsignal. The crossover control may be as described herein. The buckcrossover switch may therefore swap the inputs to buck comparator 902 atthe desired time, thereby inverting the duty cycle of the output of buckcomparator 902. After blanking circuit 903, the outputs may be used foroperating buck switches A and B. With respect to the boost portion ofthe arrangement, boost crossover switch 911 receives the duty cyclesignal, for example, an input reference voltage signal and boost rampsignal. The crossover control may be as described herein. The boostcrossover switch may therefore swap the inputs to boost comparator 912at the desired time, thereby inverting the duty cycle of the output ofboost comparator 912. After blanking circuit 903, the outputs may beused for operating boost switches C and D.

In some embodiments of the invention, control of buck crossover switchand boost crossover switch may be based on the same condition, however,this is not required, and they may be based on different signals assuitable. Moreover, it will be recognized that in the buck-boostembodiments, for example, as depicted in FIG. 8 or 9, the crossoverswitches may be disabled at certain times. For example, when thebuck-boost converter is operating in buck mode, the boost converterswitches C and D are unchanging, for example, switch C may benon-conducting and switch D may be conducting. Therefore, boostcrossover switch should not cross the switching signals to switches Cand D during buck operation. Various ways to implement this feature maybe employed. For example, each crossover switch may have an enablingcontrol that enables it only when the converter is in a mode thatrequires its operation. In another embodiment, control signal may beheld at a normal state and not crossed unless required.

In some embodiments using bidirectional switches, the bidirectionaldrive logic may direct the drive signal to an appropriate FET accordingto the polarity of the input and/or output voltage in accordance withembodiments of the invention, while the other FET may maintain lowresistance to reduce the dissipation of the reverse diode and providethe level shifting needed to drive the transistors.

FIG. 10 depicts an embodiment of the present invention using a selectiveor controlled inverting/non-inverting amplifier to implement concepts ofthe present invention. The duty cycle signal may be received at aselectively inverting/non-inverting amplifier 1020, where the selectionof inversion/non-inversion may be made by the control signal, aspreviously described, prior to being received at comparators 1001 and1012. The amplifier may work as a non-inverting amplifier or as aninverting amplifier when a proper control is activated. When suchcontrol is activated, an effect similar to that previously described forreducing effects of singularities or polarity changes.

Alternatively or additionally, some embodiments of the invention mayestablish and/or maintain stability in the PMW using various controlmethods. FIG. 11 depicts a control system diagram 1100 of a direct AC/ACconverter in accordance with embodiments of the invention. Input signalVin may be passed through a phase-locked loop PLL 1105, for example, inorder to provide a reference, and the absolute value of Vin may be takenat block 1110. This absolute value of Vin may be compared at block 1115with the absolute value of a signal derived at block 1140 from theoutput signal Vout, for example, in order to serve as sense voltage, forexample, by multiplying by a constant K at block 1135. The result ofthis comparison may be used as an error signal to determine modificationof the output signal to track the input signal. The PWM and erroramplifier behavior is represented as G(s) at block 1120. The power stagegain is represented as multiplier at block 1125. The output filter maybe represented at block 1130 as Out(s).

It will be recognized that to maintain stability in the loop, G(s) maybe calculated for optimal stabilization taking into account the powerstage gain, wherein the power stage gain is proportional to Vin. Thus,in the case of direct AC/AC conversion, for example, where Vin is asinusoidal wave changing polarity over each half cycle, the errorvoltage operated on by G(s) may reduce to zero when output voltageapproaches zero, and thus the loop correction may not work at small orsubstantially zero values of Vin. That is, the control loop may includea product that varies with Vin, which may be problematic forapplications in which constant loop gain is desired.

Accordingly, some embodiments of the invention may use a dynamic gaincompensation to maintain a constant loop gain over substantially theentire cycle of the AC voltage by canceling the effect of the inputvoltage signal. In some embodiments, the error signal may be amplifiedin an amount substantially proportional to the gain reduction of theoutput stage, thereby rendering the loop gain substantially constant,including during substantially zero input voltage signal. An absolutevalue block may be included to overcome the changes in polarity of theinput and output voltages.

FIG. 12A illustrates a circuit with dynamic gain compensation inaccordance with embodiments of the present invention. In the embodimentof the invention depicted in FIG. 12A, the effect of the power stagegain, e.g., multiplication by Vin at block 1225 may be reduced orsubstantially eliminated by dividing by Vin at divider of block 1250.Thus, the error voltage may be divided by the input voltage to maintainconstant gain such that A=Vin, where A is the power gain of the outputstage at block 1225. Vref may be a reference voltage attenuated with afactor of K at block 1235 with respect to input voltage, whereVref=K×Vin. Thus the loop gain will be:

$\begin{matrix}{{\frac{({Verr})}{Vref} \times {G(s)} \times A \times {{Out}(s)}} = {\frac{Verr}{K \times {Vin}} \times {G(s)} \times {Vin} \times {{Out}(s)}}} & (2)\end{matrix}$

Thus, based on the above equation, the term (Verr/K)×G(s)×Out(s) isindependent of Vin and thus, substantially constant gain may bemaintained over the full cycle without reference to variations in Vinfluctuations. It will be recognized that various implementations may beused to effect the above mathematical result of eliminating Vin from thecontrol loop gain, for example, by multiplying at block 1265 by theinverse of Vin produced by an inverter 1260, as shown in FIG. 12B.

FIG. 13 is a schematic illustration of one embodiment of an AC/ACconverter 1300 including several features of the present invention. Thedepicted embodiment uses a synchronous buck converter, however, it willbe recognized that embodiments of the invention may be applied to anysuitable type of power converter.

Input voltage 1305 may be used to derive a reference input voltage atnode 1310. A phase locked loop 1315, for example, using a voltagecontrolled oscillator may be used. It will be recognized that a VCO isone implementation for providing a reference voltage, but other ways arepossible, for example, using an input voltage filter. An absolute valueof the reference signal may be obtained at block 1320. An error signalmay be derived from a subtraction at block 1330 of a signal obtainedfrom the output signal, suitably scaled, for example, by a voltagedivider. This error signal may be divided by the input voltage signal,for example, by divider 1340. In some embodiments of the invention,prior to dividing, the input voltage may be limited at block 1335 toavoid dividing by zero, resulting in clipping or saturation. A Type IIor Type IR compensation-amplifier 1350 may be used to amplify thesignal, and provide it to PWM controller 1355. Crossover switch 1360,which may receive a control signal from a detector 1365, may swap theoutputs of PWM 1355 and provide the signals Qa and Qb for switches 1361and 1362. The output filter may include inductor 1370 and capacitor1375, and the load 1380 may receive the stable output voltage.

As noted above, modifications of the circuit may be used with anyconverter, for example, boost, buck-boost, Cuk, Sepic, flyback, orothers, depending on the application requirements. For a voltagestabilizer where the input voltage is approximately the same as outputvoltage, a suitable topology may be the “One Inductor” topology, forexample, using LTC3780 controller manufactured by Linear Technology.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. Forexample, the present invention may be understood to include devices,systems, and methods for directly controlling AC/AC conversion, e.g.,without converting to direct current.

In one application using embodiments of the invention, a direct AC/ACconverter may be implemented having a variable conversion ratio, forexample, for use as a transformer with constant or variable ratio. Inembodiments of the invention used for such purpose, the input/outputvoltage ratio may be changed by selectively varying the amplitude of theoutput voltage signal for the given input voltage signal. In someembodiments of the invention, the input/output voltage ratio may beselected manually, e.g., using a potentiometer or voltage divider usingelements whose impedance may be varied, which may be placed for examplebetween 1315 and 1320 of FIG. 13. In some embodiments of the invention,the input/output voltage ratio may be altered automatically, forexample, using a feedback system to cause the output amplitude to bechanged in order to follow a desired reference voltage.

Embodiments of the invention may be used for power factor correction.FIG. 14A is a schematic block diagram depicting an application for powerfactor correction using embodiments of the invention. FIG. 14 depictsdeploying an embodiment of the present invention in a three-leg unit,where an input leg to the module is connected to an input voltagesupplied to a load, an output leg to the module is connected to anoutput impedance, and the return leg is common Depending on theapplication, the output impedance may be chosen to be inductive orcapacitive, and its impedance selected. Since the AC/AC module may actas a transformer, e.g., reflecting output impedance to the input, it maybe used to adjust the amount of capacitive or inductive impedancereflected to the input, thereby enhancing the power factor. Otherconfigurations using a three-leg module embodiment of the presentinvention may also be used. For example, the connection diagram of FIG.14B may be used to enable capacitive and inductive load compensation byplacing a direct AC/AC converter module in series with the load. As afurther example, the connection diagram of FIG. 14C may be used toenable capacitive and inductive load compensation by placing a directAC/AC converter module in parallel with the load. It will be understoodthat other arrangements or connections of a direct AC/AC convertermodule may be employed consistent with the teachings of the presentinvention.

Embodiments of the invention may be used in conjunction with athree-phase output for the VCO to control three-phase direct AC/ACconverter.

Embodiments of the invention may be used for stabilizing generatoroutputs. In another application, embodiments of the invention may beused for enabling wide input voltage range AC applications, for example,motors, generators, or others. In yet another application, embodimentsof the invention may be used to enable power efficient control of lightbulbs.

Embodiments of the invention may be implemented on an integrated chip,for example, by constructing the controller, crossover switch and othercircuitry in an application-specific integrated circuit (ASIC) or otherintegrated circuit.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. A method of converting an input alternatingcurrent (AC) signal to an output AC signal using synchronous pulse widthmodulation comprising: receiving the input AC signal; producing areference signal based on the input AC signal; producing a firstpulse-width modulated signal based on the level of said referencesignal; inverting the first pulse width modulated signal, therebyproducing a second pulse width modulated signal having an inverse dutycycle; and driving a first and a second converter switches using thefirst pulse-width modulated signal and the second pulse width modulatedsignal according to a first and a second modes, thereby producing saidoutput signal, wherein in the first mode, the first converter switch isdriven by the first pulse width modulated signal, and the secondconverter switch is driven by the second pulse width modulated signal,and in the second mode, the first converter switch is driven by thesecond pulse width modulated signal, and the second converter switch isdriven by the first pulse width modulated signal.
 2. The method of claim1, wherein said reference signal is based on the phase of the inputsignal and on the level of the output AC signal.
 3. The method of claim1, wherein said first and second modes are timed with reference to azero crossing of said input signal.
 6. A method of converting an inputalternating current (AC) signal to an output AC signal using synchronouspulse width modulation comprising: receiving the input AC signal;producing a reference signal based on the level of the input signal;timely inverting the level of the reference signal; producing apulse-width modulated signal based on the level of said referencesignal; and driving at least one switch using said pulse-width modulatedsignal, thereby producing said output signal.
 7. The method of claim 6,wherein said inverting is timed with reference to a zero crossing ofsaid input signal.
 8. The method of claim 6, wherein said inverting istimed with reference to a zero crossing of said output signal.